Pearson International A Level Biology Core Practical 4: Investigate factors affecting initial enzyme reaction rate

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Measure an enzyme-controlled change over its initial linear period while controlling enzyme and substrate conditions.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use enzyme and substrate solutions, buffers, water baths, pipettes, timer, iodine or colorimetric assay equipment. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Equilibrate enzyme, substrate and buffer at the chosen condition before mixing.
2. Start timing at mixing and sample at fixed intervals or record a continuous instrumental response.
3. Repeat across a planned factor range while keeping total volume and reagent concentrations controlled.
4. Estimate initial rate from an early gradient and compare replicate means.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: temperature, pH, enzyme concentration or substrate concentration.
• Dependent variable: initial rate derived from product formation or substrate disappearance.